In a conventional reciprocating internal combustion engine, which typically operates at a relatively low engine housing temperatures (e.g. on the order of 180.degree. F.) and has acceptable low speed torque and throttling characteristics, heat is removed from the engine housing by means of a water jacket (for water cooled engines) or by metal cooling fins (for air cooled engines). Because approximately fifty percent of the heat energy created by combustion of the fuel is lost in the form of housing heat and is wasted (expelled to the atmosphere without performing mechanical work), the thermodynamic system efficiency of such a conventional engine is inherently low.
To improve efficiency of a typical reciprocating internal combustion engine in an ideal fashion, one might simply remove the radiator from the engine. The engine would then be allowed to operate at an elevated housing temperature of 350.degree. F. (current temperatures are about 180.degree. F., as noted above). At this point, steam at a pressure of about 120 psi, created in the engine housing (water jacket), could be routed into the cylinder head. Then, during the very short fraction of a second just after ignition (at the top of the power stroke) the elevated temperature (350.degree. F.) steam would be injected into the cylinder head. The combustion process, provided it is not extinguished by the steam (which is the fundamental problem), would heat the combined mixture of fuel, air and steam to about 1500.degree. F. This would provide a significant increase in the percentage of work that could then be performed on the piston during the expansion process. Namely, with the engine housing operating at the elevated temperature, pre-heated steam would be superheated by the constituents of combustion, and the total constituent working fluid would expand producing work on the piston. Unfortunately, in a conventional reciprocating internal combustion engine, this wasted engine casing heat energy is not easily recaptured to improve the engine's efficiency.
For one thing, the engine housing is not permitted to reach a temperature sufficiently hot to provide adequate potential energy to the heat transfer fluid (water as an example). Secondly, it is extremely difficult to inject the water back into the engine cylinder following the ignition and explosion portion of the cycle, but prior to the expansion portion of the power stroke. Internal combustion type engine systems which have incorporated water injection approaches have resulted in poor reliability based on the difficulties associated with timing the injection and explosion processes.
A gas turbine engine, on the other hand, which employs continuous combustion, typically does not use radiators or cooling fins. Gas turbine engines are not positive displacement engines; hence they do not have rotating blades in contact with the surface of the housing containing them. Since the rotating blades of a gas turbine engine do not come in contact with the stationary parts of the engine, the operating temperatures (typically 1300.degree. F. to 1800.degree. F.) do not cause wear problems.
Such high operating temperatures would appear to make a gas turbine engine a good candidate for improved efficiency compared to a reciprocating internal combustion engine. Indeed some gas turbines do inject water into the combustion gas stream in order to increase the power and efficiency. However, a fundamental limitation of a gas turbine engine is the fact that a gas turbine engine customarily has poor performance for low speed, high torque applications, which require throttling; adequate performance of a gas turbine engine is achieved only at very high engine speeds.